In the field of electrical distribution, it is useful to be able to measure current flowing through conductors. Often these conductors are operated at high electric potential relative to ground and thus it is advantageous to make the measurement without direct electrical connection.
It is possible to make such measurements using a current transformer. Such a device uses a magnetic circuit to couple the current in the conductor to be measured (primary conductor) and the current in a secondary conductor. This magnetic circuit typically consists of a yoke of ferromagnetic material having a conductive coil (the secondary conductor) wound therearound.
There are a number of drawbacks to this kind of current sensor. The magnetic circuit may saturate either as a result of impedance in the secondary circuit, the presence of a non-time varying current in the primary conductor, or the presence of an externally imposed magnetic field. Any of these conditions results in the current sensor losing accuracy. Furthermore, hysteresis in the magnetic circuit leads to non-linear sensor response at low operating current.
Most modern current sensors can be constructed using surface coils uniformly spaced around a central cavity through which the primary conductor passes. These are known as Rogowski sensors. Rogowski sensors include an air core, rather than a magnetic core. Surface coils include one or more turns of a conductor provided on a substrate, for example by etching or using printed circuit board technology. In order to obtain a good rejection of magnetic fields that are not due to the primary conductor, the surface coils are very precisely aligned on axes radial to a central axis of cavity through which the primary conductor passes. Such a sensor is described in U.S. Pat. No. 6,965,225 B2.